In connection with developments of digital technologies, a tendency to process or store a large amount of data at high speed has been recently promoted. Therefore, high integration and high performance have been required to semiconductor devices used in electronic equipments.
With respect to semiconductor storage devices, for example, studies on techniques using ferroelectric materials or high dielectric constant materials as a capacitance insulating film in place of a conventional silicon oxide (SiO) film or a silicon nitride (SiN) film have been started in order to implement high integration of a Dynamic Random Access Memory (DRAM).
A flash memory and a Ferro-electric Random Access Memory (FeRAM) are known as non-volatile memories in which storage information is not vanished even when power is interrupted.
The flash memory has the structure that a floating gate is embedded in a gate insulating film of an insulating gate type electric field effect transistor (IGFET), and stores information by accumulating charges in the floating gate. It is necessary to use tunnel current passing through the insulating film for writing/deletion of information, and it needs a relatively high voltage.
The FeRAM uses a ferroelectric film as a capacitance insulating film, and has a ferroelectric capacitor in which the ferroelectric film is sandwiched by a pair of electrodes. The ferroelectric capacitor is polarized in accordance with a voltage applied between the electrodes, and it has spontaneous polarization even when the applied voltage is removed. The ferroelectric capacitor also inverts the polarity of the spontaneous polarization if the polarity of the voltage applied between the electrodes inverts. The FeRAM detects this spontaneous polarization to thereby read out information. The FeRAM operates with a lower voltage as compared with the flash memory, and high-speed writing can be performed with saved power. A System On Chip (SOC) adopting the FeRAM has been considered as an application of IC cards, etc.
For example, a lead zirconate titanate (PZT), a Bi layer structure compound such as a strontium bismuth tantalite (SrBi2Ta2O9) or the like are used as the ferroelectric film used for the FeRAM. It is formed by using a sol-gel method, a sputtering method, a Metal Organic Chemical Vapor Deposition (MOCVD) method or the like. When the sol-gel method is used to form the ferroelectric film and also when the sputtering method is used to form the ferroelectric film, amorphous or microcrystal ferroelectric film is formed on the lower electrode, and then subjected to heat treatment to change the crystal structure to the perovskite structure or the Bi layer structure. When the MOCVD method is used to form the ferroelectric film, the crystal structure of the perovskite structure or the Bi layer structure is obtained in the forming process because it is formed at high temperature.
Furthermore, it is necessary that material which is hardly oxidizable or material which keeps electrical conductivity even when it is oxidized is used for the electrodes of the ferroelectric capacitor. In general, metal elements of the platinum group such as platinum (Pt), iridium (Ir), iridium oxide (IrOx), etc. and other oxides are broadly used.
It is general to use aluminum (Al) as the wire material of the FeRAM as in the case of normal semiconductor devices.
Higher integration and higher performance have been also required to the FeRAM as in the case of other semiconductor devices, and it will be required in the future to reduce the cell area. The structure of the FeRAM is roughly classified into two types of structures, a planar structure and a stack structure, however, in order to reduce the cell area, it is effective to adopt the stack structure. In FeRAM having the stack structure, barrier metal, a lower electrode, a ferroelectric film and an upper electrode are stacked immediately over a plug connected to the source-drain region of a transistor in this order to form a ferroelectric capacitor. The barrier metal serves to prevent oxidation of the plug. It has been recently frequent that the lower electrode also has the function of the barrier metal, and it has been impossible to clearly separate these elements from each other. Material such as titanium nitride (TiN), titanium aluminum nitride (TiAlN), Ir, IrO2, Pt, strontium ruthenate (SrRuO3(SRO)) or the like is used for the portions corresponding to the barrier metal and the lower electrode.
In order to form the FeRAM having an excellent electrical characteristic and a high process yield, it is important to control the orientation of the ferroelectric film constituting the ferroelectric capacitor so that the orientation is as uniform as possible. The orientation of the lower electrode for the ferroelectric film affects the orientation of the ferroelectric film materially. Accordingly, by controlling the orientation of the lower electrode so that the orientation concerned is as uniform as possible, the orientation of the ferroelectric film formed on the lower electrode can be enhanced.
For example, a method of using a stack structure of IrO2(30 nm)/Ir (30 nm)/Ti(30 nm)/TiN(50 nm) at the portions corresponding to the lower electrode and the barrier metal has been hitherto proposed (for example, see Japanese Laid-open Patent Publication No. 2005-159165).
When the lower electrode is formed of Ir and IrOx, a method of forming an Ir film at 400° C. to 550° C. and forming an IrOx film at 530° C. to 550° C. by the sputtering method (for example, see Japanese Laid-open Patent Publication No. 2001-237392) and a method using an IrO2 film in which IrO2/Ir is equal to 10 or more in X-ray diffraction intensity are also proposed (for example, see Japanese Laid-open Patent Publication No. 2002-151656). Furthermore, a method of continuously forming the IrO2/Ir stack structure by the sputtering method is also proposed (for example, see Japanese Laid-open Patent Publication No. 2000-91270).
Furthermore, (1) a method of forming an IrO2 film on an Ir film, (2) a method of forming a crystalline IrO2 film on an Ir film, forming an amorphous IrO2 film on the crystalline IrO2 film, reducing the amorphous IrO2 film in the formation process of a PZT film by the MOCVD method, and then oxidizing a reduced amorphous IrO2 film again (3) forming an oxygen-added Ir film on an Ir film, (4) forming an IrO2 film on an Ir film, forming an Ir film on the IrO2 film, and diffusing oxygen into the Ir film in the forming process of a PZT film by the MOCVD method, etc. have been also proposed (for example, see Japanese Laid-open Patent Publication No. 2003-282844, U.S. Pat. No. 6,500,678, U.S. Pat. No. 6,528,328, U.S. Pat. No. 6,548,343, U.S. Pat. No. 6,596,547, U.S. Pat. No. 6,635,497, U.S. Pat. No. 6,686,236, U.S. Pat. No. 6,872,669).
Furthermore, a method of forming an Ir film, thermally oxidizing the Ir film to form a crystalline IrO2 film on a surface layer portion of the Ir film, and changing the crystalline IrO2 film to an amorphous Ir film in the forming process of a PZT film by the MOCVD method has been proposed (for example, see Japanese Laid-open Patent Publication No. 2004-253627).
Furthermore, with respect to the method of forming the ferroelectric film, a method of conducting a Rapid Thermal Annealing (RTA) treatment under the mixture gas atmosphere of Ar gas and oxygen (O2) gas after a PZT film is formed under argon (Ar) gas atmosphere by the sputtering method, thereby crystallizing the PZT film and also forming a PZT film on the crystallized PZT film by MOCVD method has been proposed (for example, see Japanese Laid-open Patent Publication No. 2003-218325). Furthermore, there has been also proposed a method in which when a ferroelectric film is formed by the MOCVD method, the temperature is increased under an inert gas atmosphere until the temperature increases to the formation temperature, and then the ferroelectric film is formed by using gas containing O2 gas and raw material gas (for example, see Japanese Laid-open Patent Publication No. 2003-234345) and a method of changing the concentration of O2 gas between the initial stage and the later stage of the formation of the ferroelectric film (for example, see Japanese Laid-open Patent Publication No. 2003-324101).
Furthermore, a method of forming an amorphous IrOx film on the upper electrode to protect PZT film from chemical/mechanical deterioration (for example, see Japanese Laid-open Patent Publication No. 2002-261251), etc. have been also proposed.
However, it has been difficult in the conventional techniques to control the orientation of the PZT film.